1. Field of the Invention
The present invention relates to a catalyst for dimerizing at least one lower .alpha.-olefin monomer.
More particularly, the present invention relates to a catalyst for dimerizing a lower .alpha.-olefin monomer or for codimerizing two different lower .alpha.-olefin monomers, which catalyst has an enhanced catalytic activity and is useful for producing an .alpha.-olefin dimer with a high selectivity.
2. Description of Related Art
Japanese Examined Patent Publication (Kokoku) No. 42-22474 discloses a method of producing 4-methylpentene-1 by dimerizing propylene by employing a catalyst in which sodium metal is carried on a carrier consisting of a potassium compound. In this Japanese publication, as an example of the potassium compounds, potassium halides are indicated in addition to potassium carbonate. This Japanese publication, however, does not concretely teach or suggest the specific type of the potassium halides and the specific effect thereof.
Also, Japanese Examined Patent Publication (Kokoku) No. 63-25,816 indicates, as a potassium compound useful for the catalyst, potassium chloride and potassium bromide in addition to potassium carbonate. This publication, however, does not indicate a specific effect of the potassium chloride and potassium bromide.
The above-mentioned publications are completely silent as to the utilization of potassium fluoride as a component of the carrier of the .alpha.-olefin-dimerizing catalyst.
U.S. Pat. No. 4,950,632 discloses a catalyst containing, as a carrier, a mixture of potassium carbonate with potassium nitrate. Also, U.S. Pat. No. 5,081,094 discloses a catalyst containing, as a carrier, a mixture of potassium carbonate with potassium hydrogencarbonate. However, when these catalysts are employed for the dimerization of propylene, even where the selectivity of the aimed 4-methylpentene-1 is about 90%, the conversion of propylene is very low, namely, 10% to 25%. Also, potassium nitrate is disadvantageous in a high risk of explosion thereof.
Japanese Examined Patent Publication (Kokoku) No. 59-40,504 discloses a catalyst in which a mixture of sodium with potassium is carried on a carrier comprising potassium carbonate mixed with graphite. This catalyst is also disadvantageous in that the catalytic activity the catalyst is poor and/or the selectivity of 4-methylpentene-1 is not high enough for practical use.
In preparation of a homodimer of a lower .alpha.-olefin monomer or a codimer of two different types of .alpha.-olefin monomers, many types of catalysts in which a alkali metal is carried on a carrier are known. When the catalyst is employed in an industrial scale, it is advantageous to employ a compression-molded carrier in the form of grains.
Nevertheless, it is known that the conventional compression-molded carrier grains cause the resultant catalyst to be unsatisfactory in both the catalytic activity and the selectivity or in the selectivity even if the catalytic activity is satisfactory. Therefore, almost all of the conventional catalysts including the compression-molded carrier grains are not practically useful.
In the industrial preparation of the .alpha.-olefin dimers, the selectivity of the .alpha.-olefin monomer to the aimed dimer is most important and thus must be as high as possible. Accordingly, a development of a new catalyst useful for producing the .alpha.-olefin dimer with a high selectivity is strongly demanded.
As the compression-molded grain type catalyst, Japanese Examined Patent Publications (Kokoku) No. 59-40,503, No. 59-40,504, No. 59-40,506 and Japanese Unexamined Patent Publication (Kokai) No. 3-42,043 disclose catalysts in which metallic sodium is carried on compression-molded grain type carrier comprising a mixture of anhydrous potassium carbonate and carbon. Also, Japanese Examined Patent Publication (Kokoku) No. 59-40,505 discloses a catalyst comprising a compression-molded grain type carrier comprising anhydrous potassium carbonate and carbon, and a mixture of metallic sodium with potassium carbonate and carbon carried on the carrier.
When the above-mentioned catalyst is employed for the dimerization of propylene, the aimed compound, namely 4-methylpentene-1 is produced with a relatively high selectivity of 90 to 93%. This selectivity is, however, still unsatisfactory.
Japanese Unexamined Patent Publication (Kokai) No. 62-38,240 discloses a catalyst produced by oxidize-treating pellets consisting of potassium carbonate and carbon and then carrying metallic potassium on the pellets. U.S. Pat. No. 4,727,213 discloses a catalyst in which metallic potassium is carried on pellets consisting of potassium carbonate, calcium aluminate and carbon. U.S. Pat. No. 4,835,330 discloses a catalyst comprising a glass powder and metallic potassium carried on pellets comprising potassium carbonate and carbon. Those catalysts are, however, unsatisfactory because when used for dimerizing propylene, the selectivity of the resultant 4-methylpentene-1 is low, namely 90% at the highest.
In the dimerization of an .alpha.-olefin monomer, for example, propylene, the resultant reaction product contains isomers of 4-methylpentene-1, for example, 4-methylpentene-2, 2-methylpentene-2 and hexene, as by-products. Among the isomers, 4-methylpentene-2 has a boiling temperature very close to that of 4-methylpentene-1 and thus is difficult to separate it from 4-methylpentene-1 by a distillation procedure for isolating 4-methylpentene-1. This difficulty causes the resultant 4-methylpentene-1 product to exhibit a reduced degree of purity. Accordingly, there is a strong demand of providing a new catalyst effectively prevent or restrict the production of 4-methylpentene-2.
The boiling points of isomers of propylene dimers are as follows.
______________________________________ Compound Melting point ______________________________________ 4-methylpentene-1 53.9.degree. C. cis-4-methylpentene-2 56.3.degree. C. trans-4-methylpentene-2 58.6.degree. C. 2-methylpentene-1 60.7.degree. C. hexene-1 63.5.degree. C. cis-hexene-3 66.4.degree. C. trans-hexene-3 67.1.degree. C. 2-methylpentene-2 67.3.degree. C. trans-hexene-2 67.9.degree. C. cis-hexene-2 68.8.degree. C. ______________________________________